Olympics watch - the Velodrome
Engineering behind the arena, the track and the building of the Olympic Velodrome
Cycling enthusiasts do well in the Olympic Games; they are spoilt for choice. They have several events to look forward to, the BMX circuit, the road cycle pursuit, the mountain bike course, but the main events take place in the Velodrome. For the Velodrome, as for all buildings on the Olympic site, there was a lot of rubbish to be cleared away first. Work began with a 35-tonne recycling machine clearing up a 100-year-old tip; every day it separated up to 500 tonnes of industrial and domestic waste into glass, metals, concrete and soil that was used elsewhere on the site or recycled off site. Of course there were discoveries too: a medieval waterway called Hennikers Ditch ran along the route of the ancient River Leyton and passed through the site of the VeloPark. It was diverted away from the site through a specially constructed underground concrete channel. A 150 year old cobbled street was also unearthed, part of an old Templar Knights mill, experts believe. Then the real work began with the scooping out of a bowl 100m long by 60m wide and 4m deep. Then more than 900 piles 26m were driven in to give the arena a strong foundation.
All Velodromes have problems; they are on the dark side, steep banking makes it difficult for spectators to see what is going on and everyone wants to build a track that is faster than their predecessors', and it has to look good, too.
Richard Arnold, the Olympic Development Authority's project manager, is a strong advocate of 'mid-podium glazing' to let daylight in. This is how it is done. Seating is split into two sections, with 3,500 spectators at ground level and an upper tier for another 2,500. The two levels are separated at concourse level by a mid-podium walkway, which is partly internal and partly external. This concourse will house 48 concrete pillars with 48 x3m sheets of glazing, to form a continuous glass circle, a ring of light. Outside passers by can now see what going on and spectators on the inside will feel the outside buzz. And, looking ahead to the future events after the Games, separating the two tiers will minimise the effect of a dead atmosphere when the upper tier is not packed with fans.
The best seats in a Velodrome are at the sides, where the start and finish lines are, rather than at the ends where the steep banking hinders the view. The designers of the London Velodrome are aware of this and have given the roof a special steep shape to allow more seats at the sides. Disabled viewers will also benefit by the arena's unique shape. Seating areas in general have to be steep to achieve reasonable sight lines but this limits access for the disabled because the lifts that they need can block views. The 2012 Velodrome design overcomes this by adding two ramps beneath the track area that leads to the infield. The 6,000 seats are also split above and below a fully accessible public concourse that runs around the perimeter of the trackside seating, allowing wheelchair access to the best viewing points.
The Olympic Delivery Authority has a prime aim- to make the London 2012 cycling track the fastest ever and it has consulted the world's leading track designers and the United Kingdom's Olympic gold medallist, Sir Chris Hoy, to realise this. Dave Cockram, National Facilities Officer British Cycling, says Velodrome track design is a very complex geometric exercise and there are many factors that make a track 'fast'. 'The rules call for a track that is a symmetrical circuit, with two straights and two turns. In order to maintain speed on the turns they are banked, and the relationship between lengths of straight/length of turn/banking angle/radius of turn has many variations.'
'Bike riders go faster in straight lines, but in order to negotiate a track they have to change direction. This is achieved in the most efficient way by reducing the amount of extra effort necessary to make the turn. Every time a rider traverses the banking he has to counteract the reduction in speed by making extra effort. This extra effort is necessary to counteract the centrifugal effect, and to overcome the effect of the change in height the rider makes on the banking, even when riding at a fixed distance from the track edge. In effect, on every banking a rider climbs a small hill on entry to the banking and descends the hill as he leaves the banking and enters the straight. The optimum design is one that allows a rider to travel at the maximum speed with the minimum effort and the design of tracks has evolved over the years from two long flat straights with two banked circular turns into very complex geometry involving short straights with slopes of 10 degrees or more, transition curves with steadily increasing banking angles leading into circular curves with banking angles of around 45 degrees ,and transitions back into the straights of different slopes and radii.'
The ODA is also looking to thin the air and set ambient temperature and environmental conditions for fast times. They believe they can maintain higher track side temperatures to allow faster speeds and at the same time make sure that draughts are at a minimum and there is natural ventilation at the seating level so that spectators do not experience hot temperatures. However, designers have to balance sprinters' track demands against those of the endurance riders who prefer a rounder, flatter track.
The composition of the London 2012 track has not yet been finalised but a good idea may be obtained by looking at the United Kingdom's world class Velodrome in Manchester. This is a 250 metre course, supported by approximately 380 trusses with 80 kilometres of 40mm square Siberian pine as the track. (The London track may be of spruce or larch or pine). The steepest part of the track is 42 degrees and the shallowest part of the track is 12 degrees. The geometry of the design is to maximise speed. The angle of the exit from the banking is steeper than that of the entry so that a competitor is catapulted along the straights - in effect, always downhill.
David Cockram again: 'Pine and spruce are different names for the same thing. Which one you use depends on which country you are working in or buying the timber from. The timber used is a Siberian or Baltic pine or spruce that grows high in the Arctic Circle. The cold climate makes it slow growing, straight, knot free and relatively hard. The wood needs to be flexible enough to bend into the track shape and durable enough for long term use, as well as being stable in an indoor environment and not prone to splinter. Hardwoods are not used for a number of reasons, not the least of which is sustainability of source, but they don't tend to behave well in an indoor environment as they dry and splinter if attention isn't paid to humidity and temperature. Hardwoods can also be very difficult to bend into shape on a Velodrome, and they produce massive forces that distort the track as they expand and contract. '
As for the benefits of hot conditions, David Cockram says the Athens Olympics produced some very fast times in very hot conditions, and the data that was collected during training and the competition itself showed that high temperatures produced more speed. In Athens it happened naturally, the racing took place in late afternoon/evening in temperatures of 40C. Since Athens there has been a tendency to have Velodromes heated to at least 24 C for training and serious competition, and the effect has been to lower all the world record times. The downside is that spectators have to endure high temperatures, particularly high in the arena seats.
Another building with another exciting roof; this time a double curvature. Journalists compete to give a building a popular name, the Gherkin for instance, and architects are also pleased as well. Some people think the Velodrome looks like a well-known snack and have called it the 'Giant Pringle' others say that it reflects the geometry of the cycling track. Its double- curving cable net structure makes it a very lightweight building, weighing 65kg/msg, lighter than the heavier roof in Beijing of 65Kg/msg. Its size is approximately 120m by 110m wide. It will be a cable net roof, with the roof cables crossed like the strings of a tennis racket. The cables are initially laid across the infield of the Velodrome and then jacked up into position so that, as they become tensioned, the roof forms its double curved shape. Between the cables, 288 individual timber blocks or cassettes will be put in place to give substance to the roof and as the ground for the outer aluminum Kalzip seamed layers.
Legacy and Cost
After the Olympic Games have closed, the Velodrome will be kept open not only for top class cyclists but also the local community will be able to use it. The venue will also be linked into cycle routes across London. It is scheduled for completion in early 2011 and the cost will be in the region of 105m, compared with an initial 20m.
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